California is one of the most geographically and ecologically diverse regions in the world. Despite its variety of landscapes, much of California experiences a Mediterranean climate similar to other low and midlatitude regions on the west sides of continents, including the Mediterranean Basin and parts of South Africa, Australia, and Chile. All of these regions share cool winters with intermittent wetness and hot, dry summers. This type of climatic regime has characterized California for millions of years. The distinctness of this climate pattern from all nearby areas in each case makes each of these Mediterranean climate regions a bit like an island. It's allowed the evolution and assembly of many endemic species and unique ecological communities. What does a Mediterranean climate look like in California? It depends somewhat on where in the state you look. In these figures, the months of the year are along the bottom and the columns are for five cities, Sacramento in the northern Central Valley, San Francisco on the north Central Coast, Bakersfield in the southern Central Valley, San Diego on the South Coast, and El Centro in the interior southern desert near California's borders with Mexico and Arizona. The shading and the precipitation graphs shows a typical Mediterranean climate in each location, with summer droughts in the middle of the graph, typically from about May to September, and winter rains out at the tails of each graph. This basic pattern is consistent across nearly all of the state, with some exceptions far to the east, where summer rains are more frequent and important in the desert. If you think for a moment about life as a plant in California, this basic pattern presents a challenge. The warmest months of the year with the longest daylight are also the driest. Because of this, many California ecosystems have peak productivity in the early to midspring, rather than the summer. The comparison among cities shows you some important patterns. First, winter rainfall is higher in the north than in the south and higher at the coast than in the interior. Second, this temperature graph shows that it's generally warmer and summer and cooler in the winter. An important exception to this though can be when summer fog lowers temperatures right at the coast. This relates to a third overall pattern, which is that the coast has the least extreme temperature swings, while interior cities, like Sacramento and El Centro, are both hotter in summer and cooler in winter. The basic annual pattern of temperature and precipitation throughout the year is the same in nearly all parts of the state. But the variations on this theme across the state help explain the diversity of ecosystems that exist in California. Patterns of precipitation and temperature are closely tied to latitude, topography, and atmospheric conditions. The broad scale view of surface air temperature in California shows warming as you go from north to south. And precipitation generally declines from north to south as well. But there's a lot of complexity and spatial variation in both temperature and precipitation that can't be explained simply by latitude. This arises partly from the inherent structure within the atmosphere and partly because of the influence of California's varied setting and complex landscape. In particular, you can see that both winter and summer temperatures and precipitation track elevation closely. And although it's hard to see at this scale, the ocean-facing western slopes of mountains in California tend to be wetter than the leeward east-facing slopes. When moisture-laden air within storm systems sweeps over mountains, air is forced upward in a process known as orographic uplift, forming clouds and precipitation along the windward side of the mountain range. Orographic rainfall is greater on the windward side of mountain ranges and produces a dryer rain shadow effect on the leeward side. This is because winter air masses laden with moisture move from the Pacific towards the interior with the prevailing winds blowing through the northwest in California. As they're blown east, they encounter mountain ranges that force them up and over. Along the way, these air masses cool adiabatically because of the drop in pressure that they experience with increasing altitude. Cooling reduces their dew point, wringing precipitation out of them as they rise up the western slopes of California's mountains. East of the mountaintops, these air masses sink down, heating as their pressure increases, and holding on to the remaining water vapor. The rain that is wrung out of air masses rising up the windward sides of mountains is called orographic rainfall. And the phenomenon of reduced rainfall on the leeward sides of mountains is referred to as a rain shadow. In California, air masses repeatedly climb and descend mountains as they move east from the ocean, producing a general pattern of higher rainfall near the coast than in the interior. In this cross-section of Central California, you can see the decline in precipitation from the coast to Hollister, east of the first coastal range, then the decline again to Coalinga in the shadow of the interior coast range. Fresno picks up a little more moisture as air masses began to rise up the Sierra foothills. And Grant Grove experiences high levels of orographic rainfall at mid-elevations in the Sierra. At high elevations in the Sierra Nevada, natural water storage in the form of snow pack allows gradual runoff to occur and has important implications for many ecosystems. Finally, Independence lies in the rain shadow of the Sierras. And Death Valley, behind two more mountain ranges and at the lowest elevations in the Western Hemisphere, can sometimes go more than a year without any precipitation reaching it. Although we talk about annual averages, precipitation varies strongly from year to year in California. Monthly precipitation commonly ranges from less than 50% to over 150% of its long-term average. So there's really no such thing as an average rainfall year in California. Reconstruction of the past 400 years of California winter precipitation from Blue Oak tree ring analysis indicates that the high level of interannual variability observed in recent decades is not unusual. This reconstruction shows annual estimates in blue and a five-year smooth version is plotted in black. Year to year variation in California's precipitation is driven by several factors, including interannual and decadal cycles like the Pacific Decadal Oscillation and the El Nino Southern Oscillation. The North Pacific high pressure system, which normally sits between the West Coast of North America and the Hawaiian Islands, has the most immediate impact on California's climate. A network of subtropical high pressure centers, including the North Pacific High, result from the uneven heating of the Earth's surface and cause high pressure systems near midlatitudes. Instead of forming a continuous belt of high pressure, land masses cause disruptions that result in high pressure centers such as the North Pacific High. These satellite images show the position of the North Pacific High during summer and winter. And the yellow lines are isobars that show surface pressure. The North Pacific storm track becomes less active and recedes northward during summer months, while the North Pacific High is most intense and fends off incoming storms. Furthermore, descending air within the High, which warms adiabatically and increases in dew point, inhibits rainfall, creating dry summer conditions throughout California. During winter months, when the North Pacific High weakens and shifts to the southeast, the North Pacific storm track migrates southward and intensifies. As a consequence, winter storms are more intense and more likely to directly impact California because the North Pacific High is not able to block them the way it does in summer. A key feature that distinguishes California from other regions of the United States is a low-level temperature inversion. Normally, air temperature cools with increasing altitude. But below the ceiling of a temperature inversion, air temperature actually increases as you go higher. The density of air under the inversion is vertically stacked and very stable. So the low-level temperature inversion forms a cap that inhibits vertical mixing of surface air. This graph shows the vertical temperature at Oakland, California during July, with a strong North Pacific High. Above about 0.6 kilometers, air temperature declines with altitude. Below the inversion ceiling, at about 0.6 kilometers or 600 meters altitude, the opposite pattern occurs. It's colder near the ground, except for the very bottom 0.1 to 0.2 kilometers within the layer of buildings. The relatively dense air, up around 1 kilometer, traps the warm air below it, stabilizing the inversion ceiling. This both contributes to the development of an atmospheric marine layer at the coast and traps particulates and pollution. Controlled in part by the North Pacific High, these temperature inversions are a persistent feature along the California coast and inland valleys. At the coast, a coastal subsidence inversion is caused by descending air in the North Pacific High, combined with the cool air immediately above the relatively cold coastal ocean. During strong inversions, the afternoon temperature at San Diego near sea level along the coast can be 8 degrees Celsius cooler than the adjacent mountains, at about 500 meters elevation, just inland. The inversion stability also causes extensive marine stratus clouds or fog, which reflect solar radiation and moderate temperature and humidity. In addition, the higher heat capacity of the ocean compared to land means that the ocean maintains an air-conditioned coastal land zone that pervades across the coastal lowlands to locations inland where marine air can easily penetrate, moderating both high and low temperatures. Inland valleys experience inversion more often in the winter months due to lower levels of solar energy reaching the Earth's surface, cooling of land surfaces, and drainage of cool air into low-lying areas. First, fog forms over the ocean. Heating inland causes the air to rise, pulling in below it the moisture-laden marine air. Fog get sucked through the valley at the Golden Gate into the San Francisco Bay in Central Valley and also through valleys such as the Salinas Valley across the coast range, filling them also with fog. Fog largely blows with the wind from the northwest. So northern Monterrey Bay can sometimes be fog-free because the fog flows southeast and misses the bend at the bay. In spring and summer, winds in the coastal region usually blow from the northwest. These prevailing northwesterly winds can be disrupted along the Southern California Bight, the section of coastline from Point Conception to San Diego, when winds turn south during periods when the Catalina Eddy takes hold. This eddy forms in the lee of the mountainous coastline, which takes an abrupt change in direction at Point Conception. This is another way topographic complexity, via its effects on atmospheric dynamics, shapes the spatial diversity of climate in California. Next to the annual cycle, the most important pattern of climate variation in California is the El Nino Southern Oscillation or NSO. El Nino and its opposing phase partner, La Nina, are parts of a coupled ocean atmosphere phenomenon rooted in the tropical Pacific and involving fluctuations in heat content, ocean surface temperature, and trade winds. The trade winds or surface winds near the equator generally blow from east to west on the Pacific, transporting ocean surface water away from the coast of South America and driving upwelling. During normal years, sea surface temperature increases from east to west along the equatorial Pacific, with relatively cold water along the South American coast and very warm water in the western tropical Pacific. About every two to seven years, the trade winds slacken or reverse direction, increasing the sea surface temperature in the central and eastern tropical Pacific. In response, there's less upwelling along the South American coast. The resulting changes in regional heat and moisture impact global atmospheric circulation patterns. One primary impact is a shift and intensification in part of the jet stream across the northern Pacific that bring winter storms further south during winter, toward California. As a result, California can experience, but doesn't always, more frequent and intensive winter storms and precipitation during an El Nino event. In contrast, La Nina events occur when trade winds become stronger, limiting precipitation in California and generally making it dryer. Both La Nina and El Nino have the strongest effects in Southern California and fade in their effects as you go north into Northern California. Let's take a step back from the complexities of California's climate variability to explore relationships among climate, atmospheric dynamics, and air pollution in California. Remember that the temperature inversion can trap fog and low clouds, as well as particulates and pollutants. California's persistent temperature inversion can increase the effects in the state of air pollution on both health and ecological systems. Here we can see smog, trapped by temperature inversions and surrounding mountains in the Los Angeles Basin and the San Joaquin Valley. Since the mid-1940s, air quality in California has been among the worst in the nation. The infamous Los Angeles smog is a result of uncontrolled emissions from sources like industry and automobiles. Photochemical smog is produced when these pollutants react under high temperatures, light intensity, and thermal inversions, forming thousands of secondary chemical compounds, including highly toxic ozone. Air quality has improved since the 1970s because of strict air pollution measures and accompanying technological advances. In this graph, ROG are reactive organic gases and NOx are nitrous oxides, two major components of air pollution. However, emissions of particulate matter have not decreased. PM10 and PM25 are two sizes of particulate matter that remain steady in California over the last 40 years. Ozone is another important pollutant in California and other urbanized regions. Although ozone levels have improved in the state, ozone still causes serious ecological and human health effects in California. The maps show that ozone concentrations have decreased over the past 20 years, from the map on the left to the map on the right. Areas in yellow to red show ozone levels that exceed federal air quality standards. Although they are lower today, with reduced extent of exceedance levels and lower peaks, California still has the highest ozone air pollution and associated highest risks to human health in the US. Ozone damage to plants also results in decreased photosynthesis. And interactions with various other stressors influence the extent of these ozone phytotoxic effects. As these maps show, ozone can be transported long distances, affecting areas far from cities and industry. California's ecosystems are naturally nitrogen-limited. Yet chronic nitrogen deposition from industry, vehicle exhaust, and fertilizer production and use is a problem. It leads to excess nitrogen, causing acidification and eutrophication, with effects like harmful blooms in aquatic systems and plant species losses in terrestrial systems. Species adapted to low nutrient or oligotrophic environments are especially sensitive to atmospheric nitrogen. In low productivity ecosystems, such as grasslands, coastal sage scrub, and desert scrub, nitrogen deposition can enhance the growth of invasive plant species, increasing their dominance and reducing native plant species abundance and diversity. Movement and deposition of nitrogen compounds is much more spatially restrictive than ozone distribution because of nitrogen's high deposition velocity. Most nitrogenous compounds fall out onto the land surface relatively quickly and near their sources. Consequently, steep landscape gradients of nitrogen deposition occur in California mountain ranges, with the highest potential for negative effects near emission source areas like the Los Angeles region and the Central Valley. Some of these levels of background deposition, despite policies to curve nitrogen pollution, remain as high as or higher than the amounts of nitrogen that farmers put on conventional crop fields every year. Ozone and nitrogen deposition, which includes NOx or nitrous oxides, are the most ecologically damaging pollutants, while ozone and particulates pose the greatest threats to human health. But perhaps the greatest threat that these substances pose to California's ecosystem is due to their direct role in driving climate change. This photo shows the effect of the 2013 to 2015 drought on Lake Oroville, a reservoir in the South Fork of the Feather River near Chico, California. It underscores California's reliance on the climate through features like water supply. Since the beginning of the 20th century, California has warmed by approximately 0.9 degrees Celsius, which is close to the global average level of warming. Annual average precipitation over California has not changed consistently. But the warmer temperatures tend to increase the fraction of precipitation falling as rain rather than snow and also to increase evapotranspiration losses of water from ecosystems, resulting in net drier ecosystems and lower summer moisture availability regardless of precipitation amounts changing. As climate warms, many species are expanding upward in elevation or north in latitude. One such example comes from the Deep Canyon Transect, an elevational gradient in the Southern California desert that was surveyed in 1977 and again in 2007. 10 dominant plant species characterize the transect as it rises 2,000 meters from desert scrub to conifer forest. While no shifts occurred in range limits, a symmetrical shift to greater upslope cover occurred in dominant plant species. Similarly, latitudinal shifts are occurring in response to warming climates. For example, invertebrate communities of the rocky intertidal system in Monterrey Bay contain more southern species and fewer northern species than they did several decades ago. California's topographic relief increases the probability that some species will be able to track climate change While many species will be able to capitalize on this topographic opportunity, some others probably will not. Some species cannot move fast enough, or are too long lived, or will encounter soil, or competitive, or other constraints that prevent them from keeping up with shifting ranges. High elevation species could face a decrease in the area of suitable high elevation habitats available to them. And some species could literally be pushed off the top of mountains. The American pica, shown here, is a subject of much debate over whether it will adapt behaviorally and evolutionarily to climate change or disappear off the tops of California's highest mountains. In addition to considering rates of movement for single organisms, we also have to think about biomes or major types of ecosystems moving. Biomes have managed to keep up with historical velocities of climate change. But anthropogenic warming is 10 to a hundred times more rapid. The map on the left shows vegetation classes over California for a 1961 to 1990 baseline period. And the map on the right shows predictions for 100 years later. Noticeable expansions are predicted for mixed evergreen forests and grasslands. Movements today are constrained by the new challenges of extensive human development. And as organisms and biotypes shift, opportunities for species invasions and disturbances like fire could change dramatically as well. In most parts of the world, but especially in California, climate change impacts on ecosystems occur in the context of a wide range of interacting anthropogenic impacts. The impacts on California of climate changes to date are already widespread and consequential. And those in a plus 2 degrees Celsius world will be much larger. This graph shows observed and predicted temperatures relative to 1986 to 2005. Projections are for two scenarios. The blue line is a scenario with aggressive mitigation of greenhouse gas emissions. And the red line is a business-as-usual scenario. But a future with continuing business-as-usual emissions and global average temperatures in 2100 on the order of 4 degrees Celsius over preindustrial levels would be so different that the tools for projecting and describing the conditions become inadequate. The lack of information about the future, especially about the plus 4 degrees Celsius world, is heavily shaped by our almost total inexperience and inability to characterize the effects of such exceedingly fast changes in the context of a wide range of interacting ecological and anthropogenic effects. In summary, here are the main points that we've covered on the diversity of California's climates, atmospheric conditions, and pollutants and on the state's changing climate. First, California's cool, wet winters and hot, dry summers present unique challenges to organisms limited by water and temperature. Seasonal and interannual precipitation patterns in California are strongly linked to seasonal variations in the North Pacific High and in extratropical storm tracks. Third, annual precipitation amounts throughout California vary significantly over a range of time scales. Some of this variability stems from the occurrence of El Nino Southern Oscillation and La Nina events. Spatial variation across California in temperature and precipitation is strongly shaped by latitude from north to south and topography throughout the state, but especially from east to west. This spatial variation is part of what gives rise to California's ecosystem diversity. Pollutants and particles, most notably ozone, nitrogen compounds, and dust, are serious threats to ecosystems and biota, as well as humans. Temperature inversions prevent vertical mixing and trap pollutants and particles. Species responses to climate change across the state include elevation and latitudinal shifts in terrestrial and marine environments, as well as other changes described in the readings. The impacts on California of climate changes to date are ready widespread and consequential. Those in a plus 2 degree Celsius world, which would require aggressive mitigation, will be much larger. Those in a plus 4 degree Celsius world, the business-as-usual future, are really difficult to even characterize.
